forked from KolibriOS/kolibrios
213 lines
10 KiB
Plaintext
213 lines
10 KiB
Plaintext
Introduction.
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mtdbg is a debugger for Kolibri operating system. This documentation describes
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debugger features and work with it. Feel free to ask on our board (mostly
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in Russian, but has an English forum) -- board.kolibrios.org.
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General description.
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In each moment of time mtdbg can debug only one program. I will call it
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loaded program. If no program is loaded, overwhelming majority of debugging
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actions is disabled.
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mtdbg is controlled by command line, entering from keyboard. Command line
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is drawn in the bottom part of debugger window. Debugger handles standard
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input keys Backspace,Delete,Home,End,left/right arrows.
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Commands are case-insensitive. Delimiter is arbitrary nonzero number of spaces.
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At any moment mtdbg can be terminated by command "quit" (without arguments).
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You can also simply press to close button in the right upper corner of window.
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When debugger is started without command string parameters, no program is
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loaded. Also mtdbg can be started with command string, in this case it tries
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to load program with the name pointed to in first parameter in command string
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and parameters pointed to following (if present).
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If no program is loaded, you can load a program with the command
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load <full name of executable file> [<parameters>]
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Examples:
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load /sys/example
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LOAD /sys/aclock w200 h200
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LoaD /hd0/1/menuetos/dosbox/dosbox
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All that stays after first space after executable file name, is exactly passed
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to program as command string.
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The command "load" reports result in the messages window (a little higher
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than command line window). If program was loaded successfully, there will
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be the appropriate message; otherwise the message will contain error reason.
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Most probable error is "file not found" if wrong file name is given.
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The debugger can load files with information on symbols in the program
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(labels, global variables) - text files, each line of which has format
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0x<hex_value_of_addr> <name>
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(lines, which do not have such format, are ignored). Such file can be created
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by hand or generated automatically by fasm. Evident load can be done by command
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load-symbols <full name of symbols file>
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Furthermore, when the debugger executes the command "load", it checks for
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presence of file with name as of loading binary and extension '.dbg'
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(/sys/example.dbg in the first of examples above), and if such file exists,
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the debugger loads it automatically (with the message "Symbols loaded", if
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all is OK).
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It can happen so that loaded program is packed. General principle of
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program packing is following: at first input file is packed (by some
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pack algorithm), then is appended small code which gets control at program
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start, unpacks input code in the memory and then passes control to it.
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If program is packed, it "real" code is not visible and for debugging it is
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needed previously to pass through unpacker code.
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mtdbg determines most of existing packers (mxp,mxp_lzo,mxp_nrv,mtappack)
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and in this case suggests to automatically go to "real" code. It is recommended
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to accept (press 'y' or <Enter>), but you can refuse too. At refusal and if
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program is packed by something unknown the command "unpack" (without arguments)
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can be used. Call it only in the case when you are sure that program is packed
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and control has not already went to main code! [Starting from Kolibri 0.6.5.0,
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this paragraph is no more actual, because one can pack applications as all
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binary files with kpack and the unpacker code in this case is located in the
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kernel and is transparent for debug.]
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Loaded program can be terminated by the command "terminate" (without
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arguments). The command "detach" (without arguments) detaches from program,
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after that program continues execution normally, as if there was no debugger.
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After both this commands program stops to be debugged.
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It is possible to anew load program for debugging by the command "reload"
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(without arguments). If there is already loaded program, it is terminated
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and new instance is started (from the beginning) (with the same command
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string), in this case the command is similar to the commands
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terminate
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load <last program name> <last program arguments>
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Otherwise is loaded anew latest program, which was debugged (in the current
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seance of work with mtdbg) (with the same command string), i.e. is similar to
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load <last program name> <last program arguments>,
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but the command "reload" in both cases is shorter and more convenient;
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moreover, "load" thinks that new program is loaded and moves data window
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(see below) to zero address, and "reload" keeps current address.
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The command "help", which can be shorten to "h", is always available.
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All commands are divided on groups.
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"help" without arguments displays the list of command groups.
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"help" with group name displays the list of commands in this group with short
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comments.
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"help" with command name displays information about given command.
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Examples:
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help
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help control
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h LoaD
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The debugger window consists from the following items enumerated from up
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to down:
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- status string. If there is loaded program, shows its name and state
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("Running/Paused"), otherwise reports "No program loaded".
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- registers window - shows values of general-purpose registers, register eip
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and states of single flags: CF,PF,AF,ZF,SF,DF,OF: if flag is cleared, then
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is displayed lower-case letter, if flag is set, then upper-case one.
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Registers which are changed from previous moment are highlighted in green.
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- data window (dump window) - shows memory contains of loaded program
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- code window (disassembler window) - shows program code as disassembled
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instructions
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- messages window
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- command line window
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Dump window can display data starting from any address, to this serves
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the command
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d <expression>
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The command "d" without arguments flicks dump window down.
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The same is for code window and the command
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u <expression>
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or simply "u".
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Examples:
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d esi - displays data at address esi (e.g. is useful before execution of
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instruction rep movsb)
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d esp - displays stack
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u eip - disassembles instruction starting from the current
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Expressions in mtdbg can include
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- hexadecimal constants
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- names of all general-purpose registers (8 32-bits, 8 16-bits and
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8 8-bits) and register eip; values of 16- and 8-bits registers are padded
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with zeroes to 32 bits
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- four arithmetic operations +,-,*,/ (with standard priorities) and
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brackets
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- [if symbols information was loaded] names, loaded from dbg-file
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All calculations are realized modulo 2^32.
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Examples of expressions:
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eax
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eip+2
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ecx-esi-1F
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al+AH*bl
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ax + 2* bH*(eip+a73)
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3*esi*di/EAX
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The command
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? <expression>
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calculates value of specified expression.
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Values of registers in loaded program can be changed by the command "r", which
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has two absolutely equivalent forms:
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r <register> <expression>
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r <register>=<expression>
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(in both cases you can place spaces as you want). Register can be any of
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above-mentioned - 24 general-purpose registers and eip.
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Let us assume that the command "load" was successfully load program for
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debugging.
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Immediately after loading program is suspended and does not execute.
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Press Ctrl+F7 (command-line analog is the command "s") to make one step
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in loaded program, after that control returns to debugger which displays
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new contains of registers and memory. The system call "int 40h" is considered
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as one step.
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Pressing Ctrl+F8 (command-line analog is the command "p") also makes step in
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loaded program, but procedure calls, string operations with prefix
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rep/repz/repnz and 'loop' cycles are executed as one step.
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The one-step commands are used usually on single program sections,
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when it is needed, for example, to regularly trace registers value and/or
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some variables in memory.
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The command
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g <expression>
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resumes program execution and waits until control goes to eip=given address,
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and in this moment suspends program. The command "g" without arguments
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simply resumes execution.
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To suspend program use the command "stop" (without arguments).
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In the typical situation it is required that program is executed normally,
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but when some conditions are satisfied, program suspends and debugger receives
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control. The corresponding conditions are called breakpoints or simply breaks.
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Primary type of breakpoints is to concrete address, i.e. stop execution at
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eip=<given value>. Such breakpoints are set by the command
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bp <expression>
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Note that if there is only one such breakpoint, there is more convenient to use
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the command "g" with argument instead.
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Other type of breakpoints is on access to given memory area. Maximum
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numbers of such breakpoints is 4 (because hardware features of x86 processors
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are used and they allows only 4).
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bpm <expression> - breaks at any access to byte at given address
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bpm w <expression> - breaks at write to byte at given address
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bpmb,bpmw,bpmd <expression> - breaks to access correspondingly to byte, word
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or dword at given address. bpm <20> bpmb are synonyms. When bpmw,bpmd are used,
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address must be aligned according to correspondingly word bound (i.e. be even)
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or dword bound (i.e. be divisible by 4).
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bpmb,bpmw,bpmd w <expression> - similar to break on write.
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To see the list of set breakpoints use the command "bl", to obtain information
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on concrete breakpoint use "bl <number>". Unnecessary breakpoints can be
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deleted with the command "bc <number>", temporarily unnecessary can be
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disabled by the command "bd <number>", when they will be needed again,
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use the command "be <number>".
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Remarks.
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1. When debugging your own programs you can put in code instructions
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int3 (pay attention to absence of space!). Such instruction causes
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exception at normal run, which leads to process termination, but
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at work under debugger it is simply activated (with the message
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"int3 command at xxx"). This feature allows to not think about addresses
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to use in the commands g and/or bp.
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2. All output and all input is oriented on hexadecimal scale of notation.
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3. When program is executed, registers and data window shows information
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regarding to moment before resume; you can not set registers value in this
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mode. Nevertheless the command "d" in this mode shows information that
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was true in the moment of command delivery.
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diamond
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